When a control circuit having conditioned stability is controlled such as especially electrically conducted by closing a power switch or shocked by external disturbance, the control circuit tends to oscillate. "Seigyo Kohgaku" (Control Engineering), fifth Chapter, seventh verse issued by Denki Gakkai (Electric Society), Japan and written by a writer member, Ichiro Katoh says that when a control system has .omega. (angular frequency) on a Bode diagram having negative inclination ranges of third order, fourth order and less among the frequencies from 0 to the response frequency (angular frequency corresponding to loop gain =0 dB), it becomes unstable when the loop gain is lowered because the phase at the response frequency exceeds negative .pi.. Therefore, if the control system input exceeds the operation range of the amplifiers included in the control system or the normal operation range of other control components, the system oscillates because it has the condition similar to that on which the loop gain is lowered. Once it oscillates, the oscillation never stops even though the input is removed from the control system. However, if the oscillation is forced to stop by any means, it can be normally operated later. Such a condition of the control system is called conditioned stability.
In prior art, external disturbance compression ratio becomes equal to loop gain at a frequency range substantially lower than the response frequency. Therefore, the response frequency is required to be higher in order to make the loop gain higher so as to improve the external disturbance compression effect and to avoid the conditioned stability of the control system.
The maximum response frequency of an automatic control system is limited by mechanical transfer functions from a drive motor to control condition detector means such as potentiometers. In order to heighten the response frequency, the drive motor, a controlled body and the control condition detector means are required to be mechanically connected in a light and rigid manner, but it is economically and technically limited.